CT scanners, also known as Computed Axial Tomography scanners (CAT scanners), rotate a two-dimensional x-ray source around a human body. An x-ray lamp captures a plurality of flat images of the body as it moves through space in a fixed configuration. Computer-assisted reconstruction algorithms allow for digital reconstruction and the determination of useful slice images of a patient seen longitudinally from the original images taken orthogonal to the human standing axis. Since the 1930s, many types of CT scanners have been developed, such as Inverse Geometry CT, 256+ Slice CT, Dual Source CT, Multislice CT, Helical or Spiral CT, or Electron Beam CT.
At the heart of CT scanning technology lies an x-ray source acting as a radiation source operating in the wavelength in the range of 10 to 0.01 nm. These waves are capable of partial absorption by the human body. Because different parts of the body have different densities and absorption coefficients to the radiation, CT scanners can reconstruct images where the position of these different parts can be viewed from within the human body. When x-rays are shot through the body, an image is recorded on the other side of the body indicating zones where absorption was the greatest. Most CT scanners use x-ray tubes for producing the source radiation.
X-ray tubes are made of a glass bulb where a vacuum is created between an anode and a cathode separated by a small distance. By applying sufficient voltage between the anode and the cathode, electrons jump in the vacuum. In one model of x-ray tube, a layer of tungsten or molybdenum is placed upon the surface of the anode to produce degraded x-rays perpendicular to the anode surface as a byproduct of the absorption of electrons into the anode. The transformation process only produces 1% of x-rays and 99% of heat at the anode. To be powerful enough to map the human body, the x-ray tube must be placed under high voltage (from 30 to about 150 kV) and strong currents (up to 1 amperes).
High-voltage elements, if not properly insulated from their environment, can discharge current locally in a phenomenon called “arcing.” Lightning is the most obvious type of natural arcing where current from clouds at high voltage jumps through the insulating atmosphere to the earth. X-ray tubes at high voltage arc on the interior glass in an effect called “crazing” because the conductive tungsten or molybdenum migrates from the anode to the inner surface of the glass bulb and forms a conductive deposit layer. Anodes and cathodes, the elements of the system placed at the greatest energetic strain, can also arc as any high-voltage power supply for all of the reasons well known to those skilled in the art, including, for example, when partial electrical insulation weakens over time. As a consequence, CT scanners may arc at least three known locations: the x-ray tube, the anode high-voltage tank, and the cathode high-voltage tank.
Some CT scanners place the anode and cathode at opposite polarities to protect the rest of the system. For example, polarities might be set at +70 kV and −70 kV to create a voltage variation in the lamp of 140 kV. Other scanners ground the anode or the cathode and increase the voltage of the opposite element to ±140 kV. When an arc is created, the sudden, rapid discharge releases powerful energy to a different portion of the system, and as an immediate result, secondary effects are created in the system. Once an arc discharges locally, power is transferred rapidly from one of the energized component to its grounded casing or grounded shield with an associated drop in voltage for a short period of time. As the energy propagates in the structure, a series of rapid reflections are created afterward.
As electrons move and the voltage drops quickly to near zero, this variation disturbs the magnetic field located around the structure, which in turn creates a secondary current, and so forth. These secondary effects, or “ripple” effects, are known to lead to the creation of secondary arcs at different locations from the primary arc, which make detection of the location of the arc very difficult to diagnose using conventional methods.
Known technologies used to diagnose the origin of arcs include the use of sound detectors within the proximity of the x-ray tube, the anode high-voltage tank, and the cathode high-voltage tank. Since electrical discharges propagate faster than the speed of sound in air, a sonic shockwave may be created upon discharge. If the arc occurs in a solid, liquid, or semisolid, such as between layers of an insulated cable, low-frequency noise can be heard. Because CT scanners are very noisy and noise detection is often very difficult between closely positioned elements, conventional methods are unreliable to measure the primary arc and diagnose secondary arcs occurring as a result of the primary arc.
CT scanners also are equipped with their own arc detection modules. These modules cannot distinguish where arcs occur and how to differentiate between primary spit events and secondary spit events, which are defined to include but are not limited to induced arcs created as a result of the primary arc within the system.
To perform adequate maintenance on a CT scanner, the defective element must be known. What is needed is a system and apparatus to measure arcs either on the x-ray tube, the anode, or the cathode with a high degree of precision, to diagnose faults, and to predict failure of the main high-voltage components of the CT scanner.